Plastic deformation in single crystal copper

January 16, 2009

A very interesting paper from the latest Phil Mag.

Mapping mesoscale heterogeneity in the plastic deformation of a copper single crystal

K R Magid et al

Part of a ‘multiscale characterization’ study of heterogeneous deformation
patterns in metals is reported. A copper single crystal was oriented for single slip
in the (111)[101] slip system and tested to $10% strain in roughly uniaxial
compression. The macroscopic strain field was monitored during the test by
optical ‘image correlation’. The strain field was measured on orthogonal surfaces,
one of which (the x-face) was oriented perpendicular to [121] and contained the
[101] direction of the preferred slip system. The macroscopic strain developed in
an inhomogeneous pattern of broad, crossed shear bands in the x-face. One, the
primary band, lay parallel to (111). The second, the ‘conjugate’ band, was
oriented perpendicular to (111) with an overall (101) habit that contains no
common slip plane of the fcc crystal. The mesoscopic deformation pattern was
explored with selected area diffraction, using a focused synchrotron radiation
polychromatic beam with a resolution of 1–3 mm. Areas within the primary,
conjugate and mixed (primary þ conjugate) strain regions of the x-face were
identified and mapped for their orientation, excess defect density and shear stress.
The mesoscopic defect structure was concentrated in broad, somewhat irregular
primary bands that lay nominally parallel to (111) in an almost periodic
distribution with a period of about 30 mm. These primary bands were dominant
even in the region of conjugate strain. There were also broad conjugate defect
bands, almost precisely perpendicular to the primary bands, which tended to
bridge primary bands and terminate at them. The residual shear stresses were
large (ranging to well above 500 MPa) and strongly correlated with the primary
shear bands; interband stresses were small. The maximum resolved shear stresses
within the primary bands were oriented out of the plane of the bands, and, hence,
could not recover the dislocation structure in the bands. The maximum resolved
shear stresses in the interband regions lay predominantly in {111} planes. The
results are compared to the mesoscopic defect patterns found in Cu in etch pit
studies done some decades ago, which also revealed a mesoscopic dislocation
structure made up of orthogonal bands.


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